1. Field of the Invention
The present invention relates to a safety device and in particular to a device for lowering persons or diverse goods from high places. Such safety devices, including the object of the present invention, generally comprise a cable drum around which a cable or rope is wound and to an end of which cable or rope is fastened the person or load. As it turns the cable drum is braked hydraulically or pneumatically to reduce the rate of descent.
2. Description of the Prior Art
As a backgound against which to evalute the new main principles of the present invention reference may be made to the specification of U.S. Pat. No. 3,907,256 in which one such safety device is described. A characteristic of devices of this kind is a rotating cylinder which is adjustably coupled to the cable drum when the speed of said drum is sufficiently high. The clinder is mounted on a body and it contains a central piston, which is fixed to the cylinder and acts as a partition dividing the cylinder into two chambers. The device additionally has two rotating pistons and end walls to the cylinder. THe end walls of the cylinder and all the pistons have a "wavy" form. The safety device of this patent is further characterized in that it has one cylinder which is closed and filled with a fluid, and when the device is in operation flow of said fluid takes place on both sides of said central fixed piston from one side to the other.
Whatever the merits of the aforesaid known safety device are, it also has some serious drawbacks. In particular, because one of the main principles of its operation is the flow of fluid from one side to the other of the "wavy" shaped pistons, these pistons as a consequence of their special shape are not tight. Because of this lack of tightness of the pistons and also of the axle which passes through the whole cylinder, it is not possible to determine with certainty the exact rate of descent which will be provided by the device. Secondly, the rapid motion of the pistons causes a change in the viscosity of the fluid, which in turn causes a reduction in the braking effect of the fluid and thereby an increase in the rate of descent. Moreover, the rapid motion of the pistons causes appreciable wear in them, so that in hard use there is an evident danger of seizure. Further, the replacement of valves by orifices, which was characteristic of the previously mentioned device, requires the use of thick oils for the fluid of the braking mechanism and this in turn makes the device dependent on weather conditions.
Another known safety device is that described in U.S. Pat. No. 4,173,332 which differs from that described above in that the cylinder is fitted with only one piston, said piston being a close fit in the cylinder and moving back and forth in an axial direction, which axial motion is brought about by means of the interaction of a sinusoidal groove around the circumference of the rotating piston and pins fitted to the fixed cylinder so as to engage in said sinusoidal groove. The operation of the device is similar to that of the device described previously and its drawbacks are also the same.